LEDs with an overall high luminance are useful in backlighting for Liquid Crystal Display (LCD) based monitors and televisions, collectively hereinafter referred to as a matrix display. In a large LCD matrix display, typically, the LEDs are supplied in one or more strings of serially connected LEDs, thus sharing a common current. Matrix displays typically display the image as a series of frames, with the information for the display being drawn from left to right in a series of descending lines during the frame.
In order to supply a backlight for the matrix display, one of field-sequential lighting and non-field-sequential backlighting is employed. In non-field-sequential backlighting, either one or more strings of “white” LEDs are utilized as a luminaire, the white LEDs typically comprising a blue LED with a phosphor, which absorbs the blue light emitted by the LED to emit a white light, or one or more individual strings of colored LEDs, functioning as a luminaire, are placed in proximity so that in combination their light is seen as white light. In a field-sequential system, colored LED strings are exclusively utilized, and the colored LED strings typically are lit sequentially. The LCD is synchronized with the sequential lighting of the colored LED strings. In effect, at least three images are displayed for each frame; a red image, a green image and blue image, with the colored images being displayed sequentially. Such a field-sequential system is described in U.S. Patent Application Publication S/N US 2006/0097978 A1, published May 11, 2006 to Ng et al, the entire contents of which is incorporated herein by reference.
A field sequential system advantageously does not require color filters for each of the pixels, since the color is directly supplied by the backlight LEDs. As a result, an increased percentage of the light produced by the backlight LEDs is transmitted through the LCD and perceived by the viewer. Unfortunately, this advantage leads to certain difficulties in color control.
The human eye has receptors responsive to different wavelengths of light. In particular, it is understood that three different types of receptors are typically found, each associated with a certain wavelength. Thus, in order to completely describe a color sensation, i.e. brightness, hue and saturation, at least three values are required. The three values may be described in any of a plurality of color spaces, also known as colorimetric systems, which may be standardized color spaces such as the CIE 1931 color space, or an RGB color space associated with a particular color sensor. Translation between color spaces is typically accomplished mathematically, with translation between standardized color spaces being accomplished in cooperation with known fixed values, and translation between particular color spaces and standardized color spaces typically requiring certain calibration information. The term tri-stimulus values as used herein, is meant to include any set of values which represent the luminance and chromaticity of a color sensation, in any color space.
In particular, colored LEDs change their luminance and hue characteristics as a function of age and temperature. Typically, a color sensor is thus provided, in optical communication with the source lighting. The output of the color sensor is fedback to a controller or manager, which is operative to adjust the drive signals of the respective colored LEDs so as to achieve a pre-determined color temperature. In non-field-sequential backlighting, where more than 90% of the light produced by the backlight LEDs is not transmitted through for perception by the viewer, ambient light is similarly strongly attenuated prior to reaching the color sensor. As a result, changes in ambient light do not currently significantly impact the color control of a non-field-sequential backlight.
As indicated above, in a field-sequential system, optical attenuation between the LEDs and the viewer is significantly reduced. As a result, the impact of ambient light on the color sensor used to control the colored LEDs is increased. The impact of ambient light can have negative consequences in regard to the color control loop. In a first consequence, the ambient light impacts the luminance values of the color control loop. Thus, increased ambient light is read as increased luminance from the LEDs, resulting in decreased output of the LEDs responsive to increased ambient light. Unfortunately, this is the opposite of what is desired, since with increased ambient light an increased light output is desired to prevent washout of the image.
In a second consequence, the color of the ambient light impacts the chromaticity values of the color control loop. Thus, in an environment in which an ambient light of a certain hue is experienced, the color control loop will reduce the color components associated with ambient light hue. This is similarly unfortunate, as the resultant colors associated with the ambient light hue will be washed out.